The present invention relates to a servo control device for a VTR employing a microprocessor.
There is conventionally known, in Japanese Patent Laid-open Publication No. 58-186274, for example, a control system employing a microprocessor for controlling a drum servo, a capstan servo and a reel servo according to operation modes of a VTR.
As shown in FIG. 6, the above control system includes a single microprocessor 30. The microprocessor 30 is connected through a data bus 31 to a reel servo 32, a capstan servo 34, a scanning drum servo 36 and a machine communication interface 38, so that various operation modes of the VTR are controlled according to commands from a user or by remote control.
By way of example, interrupt request signals (IRQ) to microprocessor 30 are output from drum servo 36.
A reference generator 40 generates a system clock on the basis of a reference office composite synchronizing signal supplied thereto through a line 42, and synchronizes clock timing of microprocessor 30 and the servo circuits.
Microprocessor 30 is connected to an automatic scanning tracking servo 44 and a tape synchronizing processor 46. A tape composite synchronizing signal is supplied through a line 48 to processor 46, and an output from processor 46 is supplied to a time base correction interface 50.
When a single microprocessor is employed as mentioned above, the following problems occur with the servo circuits.
In the drum servo circuit, a phase servo and speed servo are concurrently employed. Feedback is carried out in the phase servo at the rate of rotation of a drum motor and at several times the rotational rate in the speed servo.
Accordingly, a speed detection interrupt routine is carried out by the microprocessor several times during each drum motor rotation. Therefore the higher the rotational speed of the drum motor, the more frequently interrupt routine is performed, with an increasing burden on the microprocessor.
In an ordinary analog VTR, the rotational speed of the drum is low, typically 30 rps. Accordingly, even when the interrupt routine is carried out six times per rotation of the drum, the interrupt period is relatively long, i.e. 5.5 mS.
However, in a D-1 type (also called a 4:2:2 type) VTR digital recording of a component type video signal (Y, R-Y, B-Y), the rotational speed of the a drum is set to a relatively high rate such as 150 rps. Accordingly, if the interrupt routine is carried out six times per rotation of the drum, for example, the period is shortened to 1.1 mS, causing an increase in the burden on the microprocessor.
In the capstan servo circuit, the rotational speed of the capstan motor is low, typically no more than 20 rps. Accordingly, even when an interrupt is carried out six times per rotation of the capstan motor, an the interrupt period is becomes relatively long, i.e. just over 8 mS, thus not causing any serious problems.
In the reel servo circuit, it is necessary for the response of the tension servo to be as rapid as about 100 Hz. Further, it is necessary to carry out feedback at the rate of 500 times/S or more, for example, in order to also reduce phase lag up to 100 Hz. In other words, interrupt processing with a short period of 2 mS, for example, is required, which causes an increase in the burden on the microprocessor.
Further, when three kinds of servos are processed by a single microprocessor as mentioned above, the interrupt frequency is increased. As a result, the burden on the microprocessor is further increased, and interrupt processing overlaps more causing a process waiting state, with the result that interrupt processing cannot be carried out at a predetermined timing thereby reducing servo performance.
To avoid reduction in servo performance as mentioned above, it may be considered to control the three kinds of servos by means of respective independent microprocessors.
However, in this case, the reel servo and the capstan servo, which are closely related to each other in terms of tape movement control, are undesirably separated. As a result, there is a problem with tape movement control, in which selection between reel movement mode and capstan movement mode becomes uneven and slow, or acceleration and deceleration during capstan operation become slow.